Add the sugar and butter to a thick based pan over a medium heat and melt together, swirling the pan instead of stirring it. Once the 2 are well combined allow it to simmer and caramelise further, turning a rich brown colour. Remove the pan from the heat then add the thyme leaves and stir through to combine. Place the peach halves in the pan with the cut side facing up. Gently roll out the pastry, being careful not to roll the dough out too much. Cover the peaches with the pastry, tucking the sides or any excess pieces of pastry into the pan. Place the pan in an oven that has been preheated to 200C to bake for 20 minutes or until the pastry is golden brown and has puffed up. Remove from the oven and allow to cool slightly. While the tart is baking prepare the mascarpone cream by combining all the ingredients together in a mixing bowl. Using an electric hand mixer, begin mixing the ingredients together until combined.Carefully place a serving dish over the tart and turn over being careful not to let let the hot caramel run out. Cut into slices and serve with a spoon of the mascarpone topping the tart.